Joomla explicitly prohibits any two user accounts to use the same email address.
However, there are many community clusters that have a need to share and use a single email address. For example, church websites that want to support their constituents have issues with elderly members and families that typically do not have an email address for each individual.
Trying to modify Joomla (hacking) is not easy (no one has really succeeded) and is not recommended.
Creating a unique email address for each family member is an option that at least overcomes the (imo well justified) Joomla restriction. But in most cases this will just generate overhead for the Internet literate family member having for example to manage email address for other family members (elderly, infants, etc.).
>>> But there is a solution that does not hack Joomla and that makes things manageable also.
It is called email sub-addressing using +tag qualifier.
Most email servers support this (tested gmail, hotmail and our email server) little known cool feature.
How does it work?
Lets say you have an email address email@example.com. With email sub-addressing the following email addresses are valid and of course unique for Joomla:
Any email sent to one of the above addresses will be delivered to the firstname.lastname@example.org account but the to: address will retain the +tag part (so the email manager knows which tag was used).
Try it - it works and provides a great service for many communities that want to use Community Builder and Joomla for their websites and have hit the unique email address wall.
More about email and address tagging can be found on wikipedia.